Joe Leibowitz figures he has about a year to go as proprietor of the grungy but delightful little candy store on the corner of 75th and Columbus. His rent now is $1,000 a month -- for a space approximately 12 feet wide -- and it will probably double as soon as his lease expires.

In his East-European accent, Leibowitz says: "Something has to give" -- and he means himself.

He means that he will move off Columbus Avenue or farther up on Colombus Avenue and turn this cherished space back to its landlord, who will probably have no trouble finding a maker of natural, organic, vitamin-enriched, low-calorie pasta who will race to be first to plunk down $2,500 a month for the prize of 12 feet of frontage.

Whether New Yorkers view this development with enthusiasm or melancholy depends mostly on whether they own property on Colombus Avenue. Or whether, instead, they have a taste for reading that surpasses the attention span required for the latest issue of People magazine. As things are currently progressing on Columbus Avenue, the readers are losing and the landlords are winning.

In New York, of course, the traditional neighborhood candy store is not really a candy store at all, but a sort of gathering point where residents can buy newspapers, magazines, swap insults or gossip, and possibly find a cup of coffee that still costs a quarter.

Joe Leibowitz's magazine rack offers esoterica (and some erotica) not available on shelves for many blocks around, but that is not really the point. The point is how neighborhoods in the city change and why.

A real-estate boom is said to be in full explosion now in New York or more particularly in Manhattan. It is nowhere more evident than on the island's Upper West Side; which for generations was the side of town where rent was cheaper and the atmosphere more relaxed -- compared, that is, with the East side, which is commonly described as posh in wire service stories and "up tight" by Westsiders who consider themselves on the fringes of bohemia, which may merely mean they are less likely to be found working 9 to 5 hours.

All that has been changing along Columbus Avenue, particularly in the last five or six years.

"The neighborhood is basically upward mo" said Jack Donias, co-owner of Ruskay's, one of the most-popular restaurants on the street.

"Upward mo" -- taken from the term upward mobility -- in New York or elsewhere carries with it an implication of homogeneity. Residents begin to look uniformly young, uniformly fashionable uniformly striving.

One of the basic problems, with your basically upward mo neighborhood, one that is not always foreseen, is that it becomes increasingly difficult to buy toothbrush or anything else useful in this new ghetto of consumers, where most people seem to shop with a copy of The New York Times "Living" section under their arms.

A man could drown in a sea of Perrier water and Red Zinger iced tea (looking very stylish in his rumpled linen Italian sportscoat as he floats past the fern fronds and the tables of curious brunchers) trying to find a place that can sell him some denture adhesive.

"Just what we need," said Leibowitz a few months ago when one of the last hardware stores on the street closed its doors to make way for the remodeling crew, "another expensive restaurant."

Thirty years ago, Columbus Avenue was known as "tough Irish," and the Jewish parents who lived over on West End Avenue, two blocks away, warned their children to be careful crossing Columbus or to stay away from it altogether. The property became increasingly rundown. Poor blacks and Puerto Ricans moved in. Much of the Irish population moved out. The neighborhood retained its reputatton as a rough part of town.

The first real change came to Columbus Avenue 15 years ago with the development of the Lincoln Center for the Performing Arts, which cleared a swath of West Side slums and provided a sort of anchor for the neighborhood. Gradually, the renewal spread northward up Columbus Avenue from 66th Street and was helped greatly by a real-estate tax abatement plan that encouraged landlords to renovate the residential property in the area. The tax break was desired as a tool to encourage middle-class residents to stay in the city, because the savings to the landlords, were supposed to be passed on to the tenants in the form of lower rents.

The plan worked exceptionally well, at least in the beginning. But in recent years, as property has been renovated, rents on the West Side, from 66th Street to 86th Street have skyrocketed, and apartments now are scarce.

Four years ago, a fairly spacious one-bedroom apartment in a newly renovated brownstone on the Upper West Side could be had for about$350 a month. The same apartment now goes for $550 to $600, and more and more people are trying to hang on to their apartments -- even at such prices, because virtually any move is going to mean a rent increase.

And on Columbus Avenue the axis for this trend has shifted from the aggressively funky to the fashionable. The street is lined with antique shops and restaurants. There isn't a drugstore left on the avenue between 72nd and 83rd Streets.

The first commercial casualties on the street were those whose profit margins were erased with the first great leaps in the rent -- the mon-and-pop groceries, the hardware stores, drugstores and stationery shops whose proprietors could find no way to raise prices or increase volume high enough to stay in business.

Residents who had been in the neighborhood longest watched with a curious mixture of pride and alarm. The pride came from the clearly visible physical improvement of the area. Along the side streets, big dumpsters, three car lengths long waited to haul away the plaster and splintered wood from brownstone renovation projects. Cute flower shops opened on the avenue, and sidewalk cafes began to flourish in the summer-time evenings. There seemed to be more people on the streets with each passing week and an air of bustle and activity in the evenings. Even the dope dealers, who for years held the corner of 73rd and Columbus as a kind of shadowy bazaar offering a complete pharmacopeia of chemical intoxicants, vanished without a trace. Their market, apparently, dried up. (It was not that some of the new residents were not consumers of drugs; they were just more likely to purchase their cocaine at the office.)

The elderly of the neighborhood, although still in evidence, were being rapidly moved out. A sizable gay population began to move in, both as residents and entrepreneurs.

There seemed to be no limit to the number of antique and nostalgia shops the neighborhood could support, or to the number of $5 omelets it could sell on any given Saturday or Sunday morning.

"It is positive?" asked Bernard Walpin, a real-estate lawyer who works with Subud Realty, one of the large real-estate agencies on the West Side. "Of course it's positive! The exodus to the suburbs has been reversed. It's a little stream that's turned into a torrent. The average person's assumption of what a neighborhood is like is almost two to three years behind the times. Now people are catching on to what has been going on here on the West Side.

"People always used to assume that the West Side was not the proper place to be. Real estate here was tremendously undervalued. Now it seems high, but it will level off."

People in the market for apartments on the West Side go to Subud Realty and similar agencies hoping -- for a fee that can range up to 15 percent of the first year's rent -- to find an apartment they can afford. Subub has a reputation for specializing in large apartments in older apartment buildings, the kind found along Riverside Drive, West End Avenue and Central Park West. But this summer, there are fewer ads in the papers, and Subub has few apartments to show.

"Prices are making people stay put," Walpin said. "It's getting so that you can't find a five-room apartment on Central Park West for under $1,500 a month."

All around the West Side, you can see apartment hunters parading the streets with the classified ads folded under their arms. Few of them look happy.

"I just looked at a damn closet for $500," said Mary Gibbon, a young woman who has just moved here from Indianapolis. "My furniture is in storage and I'm living with a friend. I knew rents were high here, but I had no idea they were this bad. I'm desperate. I've got to find something soon. People told me that the Upper West Side was cheaper than most places in the city, but I'm beginning to wonder. I guess I'll have to go farther north."

The people who used to live along Columbus Avenue streets numbered in the 60s and 70s are already one step ahead of Mary Gibbon. When one walks north along Columbus, into the 80s, the movement is visible. Month by month, the improvement in the real estate can be seen advancing up the avenue and spreading along the residential streets that cross it.

For example, a large apartment building stands on Columbus between 81st and 82nd Street. It has more than 400 rooms. For years, the building, its walls apparently as solid as the pyramids, was a tremendous problem to the neighborhood. Through the late 1960s and early 1970s, the building, the Endicott Hotel, was a welfare apartment house, known for its squalor and violence. The building was condemned for a while and boarded up. Junkies broke in and lived there, causing occasional fires and spasmodic demands from residents to do something about the problem.

Last year the building was sold and a massive renovation project is now under way. Sitting as it does at the back corner of the Museum of Natural History and the Hayden Planetarium, it is in a highly desirable location. When it is finished, apartments in the building will go for premium rates, even by West Side New York standards. And when the building is occupied, it is likely to bring an immediate change to the stretch of Columbus Avenue that borders it.

Columbus Avenue in the blocks around the 80s still has a drugstore and a shoe repair shop and Puerto Rican bodegas (small grocery shops) that sell green bananas and disposable diapers. Here and there a couple of antique shops have an early toehold, and there is already a restaurant upon which the food critic of The New York Times has bestowed a one-star rating.

Meanwhile, landlords are checking the expiration dates on their leases with high anticipation, and progress continues unabated.